Sure, you could go to some sketchy black market auction and buy one, but if you want to make sure you get exactly what you want, it’s best to find a new slave and break him in yourself.
It’s a lot more work to start from scratch, but when you control of every stage of training, you can get exactly the behavior you want without having to undo someone else’s work. Of course, a new slave will often be resistant to his change in status, so initially you’ll need to be ready for him to put up a hell of a fight.

